Method of forming a turbine blade with cooling channels

ABSTRACT

A method of constructing a turbine blade for a gas turbine engine is provided. The method includes casting the blade, forming a plurality of spaced-apart notches in the airfoil proximate the tip on the pressure sidewall and forming at least one hole in each tip shelf communicating with the interior void of the airfoil for channeling cooling air from the interior void of the airfoil to thereby form a squealer tip.

This is a divisional application of U.S. patent application Ser. No.10/970,031, filed on Oct. 21, 2004, now U.S. Pat. No. 7,270,514.

TECHNICAL FIELD AND BACKGROUND OF THE INVENTION

The present invention relates generally to turbine blades for a gasturbine engine and, in particular, to a method of rebuilding the tips ofcontinuous incline turbine blades, and the resulting rebuilt blade.

It is well known that air is pressurized in a compressor of a gasturbine engine and mixed with fuel in a combustor to generate hotcombustion gases. These gases flow downstream through one or moreturbines where energy is extracted. In a typical turbine, a row ofcircumferentially spaced-apart rotor blades extend radially outwardlyfrom a supporting rotor disk. Each blade typically includes a dovetailwhich permits assembly and disassembly of the blade in a correspondingdovetail slot in the rotor disk, as well as an airfoil which extendsradially outwardly from the dovetail.

The airfoil has a generally concave pressure side and generally convexsuction side extending axially between corresponding leading andtrailing edges and radially between a root and a tip. The blade tip isclosely spaced in relation to an outer turbine shroud for minimizingleakage of the combustion gases flowing downstream between the turbineblades. Maximum efficiency of the engine is obtained by minimizing thetip clearance, but is limited by the differential thermal and mechanicalexpansion and contraction coefficients between the rotor blades and theturbine shroud. Sufficient spacing must be maintained in order tominimize the occurrence of undesirable contact between the blade tip andthe turbine shroud.

The turbine blades are bathed in hot combustion gases, and effectivecooling is therefore required in order to extend the useful life of theblade. The blade airfoils are hollow and disposed in flow communicationwith the compressor so that pressurized air may be bled from thecompressor and used in cooling the airfoils. Airfoil cooling is quitesophisticated and may be effected using various forms of internalcooling channels and features, as well as cooling holes through thewalls of the airfoil for discharging the cooling air.

The blade tip is particularly difficult to cool since it is locateddirectly adjacent to the turbine shroud and the hot combustion gaseswhich flow through the tip gap. Accordingly, a portion of the airchanneled inside the blade is typically discharged through the tip forcooling. The tip typically includes a continuous radially outwardlyprojecting edge rib formed coextensively along the pressure and suctionsides between the leading and trailing edges. The tip rib follows theaerodynamic contour around the blade and is a significant contributor tothe aerodynamic efficiency of the blade.

Generally, the tip rib has portions spaced-apart on the oppositepressure and suction sides to define an open top tip cavity. A tip capextends between the pressure and suction side ribs and encloses the topof the blade for containing the cooling air. Tip holes are also providedwhich extend through the floor for cooling the tip and filling the tipcavity.

Several patents relate to the cooling of turbine blade tips, including:U.S. Pat. No. 5,261,789 to Butts et al.; U.S. Pat. No. 6,179,556 toBunker; U.S. Pat. No. 6,190,129 to Mayer et al.; U.S. Pat. No. 6,059,530to Lee and U.S. Pat. No. 6,672,829 to Cherry et al. These patentsdisclose various blade tip configurations which include an offset on thepressure and/or suction sides in order to increase flow resistancethrough the tip gap.

In particular, U.S. Pat. No. 6,672,829 discloses a turbine blade tipthat alters the pressure distribution near the tip region to reduce theoverall tip leakage flow and thereby increase the efficiency of theturbine. The blade tip develops a recirculation zone adjacent the ribsin order to improve the flow characteristics and pressure distributionat the tip region. This is accomplished by means of an inclined squealerand a shielded film shelf. It is anticipated that the tip region of theblade will require several rebuilds during the service life of theblade. A practical way of carrying out these rebuild procedures isnecessary to achieve the full advantage of this new blade. Thisapplication discloses the use of electrical discharge machining (EDM) tocarry out this rebuild process, and the resultant turbine blade withdiscrete film shelf notches.

BRIEF DESCRIPTION OF THE INVENTION

In one embodiment of the invention, a turbine blade for a gas turbineengine is disclosed, and includes an airfoil and integral dovetail formounting the airfoil along a radial axis to a rotor disk inboard of aturbine shroud. The airfoil includes first and second sidewalls joinedtogether at a leading edge and a trailing edge, where the first andsecond sidewalls extend from a root disposed adjacent the dovetail to atip cap for channeling combustion gases thereover. At least one tip ribextends outwardly from the tip cap between the leading and trailingedges. The turbine blade tip on the pressure side is provided withmultiple discrete open notches.

In an embodiment of the method of the invention, a turbine blade for agas turbine engine is disclosed and includes an airfoil and integraldovetail for mounting the airfoil along a radial axis to a rotor diskinboard of a turbine shroud. The airfoil further includes first andsecond sidewalls joined together at a leading edge and a trailing edge,where the first and second sidewalls extend from a root disposedadjacent the dovetail to a tip cap for channeling combustion gases. Inrepairing the blade tip, the damaged squealer is ground off andreplacement material is welded into place. A wedge-shaped electrode isused with an EDM process to form wedge-shaped notches on the newsquealer. Film cooling holes are drilled into the shelf of the notches.

According to one aspect of the invention a method of rebuilding aturbine blade for a gas turbine engine is disclosed, the blade being ofthe type including an airfoil having first and second spaced-apartsidewalls defining an interior void and joined at a leading edge and atrailing edge. The first and second sidewalls extend from a rootdisposed adjacent the dovetail to a tip cap for channeling combustiongases thereover, and a squealer tip including at least one tip ribextends outwardly from the tip cap. The method includes the steps ofremoving the squealer tip, including the at least one rib tip, from thetip cap and adding new material to the tip cap to serve as a newsquealer tip. A plurality of spaced-apart notches are formed in the newmaterial between the leading edge and the trailing edge of the airfoil,and at least one hole is formed in each notch communicating with theinterior void of the airfoil for channeling cooling air from theinterior void of the airfoil to thereby form a squealer tip.

According to another aspect of the invention, the step of removing thesquealer tip from the airfoil includes the step of grinding the squealertip off of the airfoil. According to another aspect of the invention,the step of adding new material to the tip cap to serve as a newsquealer tip comprises the step of welding at least one metal blank ontothe tip cap.

According to another aspect of the invention, the step of forming aplurality of spaced-apart notches in the new material between theleading edge and the trailing edge of the airfoil comprises the steps ofproviding EDM electrodes having a predetermined shape suitable forforming the notches, applying the EDM electrodes to predeterminedpositions on the airfoil, and utilizing the EDM electrodes toelectrically discharge machine-form the notches into the new material.

According to another aspect of the invention, the step of forming atleast one hole in each notch comprises the step of drilling the hole.

According to another aspect of the invention, the step of forming the atleast one hole comprises the step of drilling a plurality of holes ineach notch.

According to another aspect of the invention, the step of forming aplurality of spaced-apart notches in the new material comprises thesteps of providing EDM electrodes having a predetermined shape suitablefor forming the notches, and applying the EDM electrodes to the newmaterial in a spaced-apart, longitudinally-extending array between theleading edge and the trailing edge of the airfoil proximate a top edgethereof. The EDM electrodes are utilized to electrically dischargemachine-form the notches into the new material.

According to another aspect of the invention, a method is provided forconstructing a turbine blade for a gas turbine engine, the blade beingof the type including an airfoil having first and second spaced-apartsidewalls defining an interior void and joined at a leading edge and atrailing edge, the first and second sidewalls extending from a rootdisposed adjacent the dovetail to a tip cap for channeling combustiongases thereover and a squealer tip including at least one tip ribextending outwardly from the tip cap. The method comprises the steps ofmetal casting the blade, including the airfoil, forming a plurality ofspaced-apart notches in the airfoil proximate the tip, and forming atleast one hole in each notch communicating with the interior void of theairfoil for channeling cooling air from the interior void of the airfoilto thereby form a squealer tip.

According to another aspect of the invention, the step of forming aplurality of spaced-apart notches comprises the steps of providing EDMelectrodes having a predetermined shape suitable for forming thenotches, and applying the EDM electrodes to predetermined positions onthe airfoil. The EDM electrodes are utilized to electrically dischargemachine-form the notches into the airfoil.

According to another aspect of the invention, the step of forming aplurality of spaced-apart notches in the airfoil comprises the steps ofproviding EDM electrodes having a predetermined shape suitable forforming the notches and applying the EDM electrodes to the airfoil in aspaced-apart, longitudinally-extending array between the leading edgeand the trailing edge proximate a top edge thereof. The EDM electrodesare utilized to electrically discharge machine-form the notches into theairfoil.

According to another aspect of the invention, a turbine blade for a gasturbine engine is provided, comprising an airfoil having first andsecond spaced-apart sidewalls defining an interior void and joined at aleading edge and a trailing edge. The first and second sidewalls extendfrom a root positioned adjacent a dovetail to a tip cap for channelingcombustion gases thereover. A squealer tip extends outwardly from thetip cap, and comprises a plurality of spaced-apart notches formed in theairfoil proximate the tip cap and between the leading edge and thetrailing edge of the airfoil. At least one hole in each notchcommunicates with the interior void of the airfoil for channelingcooling air from the interior void of the airfoil.

According to another aspect of the invention, each notch is recessedwith respect to the first sidewall to form a tip shelf.

According to another aspect of the invention, the squealer tip isintegrally-formed with the airfoil.

According to another aspect of the invention, the squealer tip is formedfrom material welded to the tip cap.

According to another aspect of the invention, the squealer tip includesa spaced-apart, longitudinally-extending array of notches positionedbetween the leading edge and the trailing edge of the airfoil proximatethe tip cap.

According to another aspect of the invention, the at least one hole isformed in the tip shelf of the notch.

According to another aspect of the invention, a pair of holes are formedin the tip shelf of each of the plurality of notches, the tip shelfbeing substantially normal to the radially-extending axis of the bladeand each of the notches having a width in substantial alignment with thewidth of the airfoil and a height along the radially-extending axis ofthe blade, wherein the width is greater than the height.

According to another aspect of the invention, the first sidewall is apressure side sidewall, and the notches are formed in the pressure sidesidewall.

BRIEF DESCRIPTION OF THE DRAWINGS

Some aspects of the invention have been set forth above. Other aspectsand advantages of the invention will appear as the invention proceedswhen taken in conjunction with the following drawings, in which:

FIG. 1 is a perspective view of an exemplary gas turbine engine rotorblade disk having a tip in accordance with the teachings of U.S. Pat.No. 6,672,829;

FIG. 2 is an fragmentary enlarged view of the blade tip illustrated inFIG. 1;

FIG. 3 is an elevational, sectional view through the blade tipillustrated in FIG. 2, taken generally along line 3-3, and depicting amaximum angle between a longitudinal axis through the blade tip ribs andthe radial axis;

FIG. 4 is an elevational, sectional view through the blade tipillustrated in FIG. 3 within the turbine shroud, taken generally alongline 4-4, and depicting a minimum angle between a longitudinal axisthrough the blade tip ribs and the radial axis;

FIG. 5 is a cross-sectional view of the blade tip with the damaged orworn squealer ground off;

FIG. 6 is a cross-sectional view of the blade tip with additionalsquealer material added by welding to the top of the blade tip;

FIG. 7 is a cross-sectional perspective view of the blade tip showingthe position of the EDM electrodes;

FIG. 8 is a cross-sectional perspective view of the blade tip showingthe formation of the tip squealer and film shelf;

FIG. 9 is a cross-sectional perspective view of the blade tip showingthe formation of the film cooling holes in the tip squealer and filmshelf;

FIG. 10 is an fragmentary enlarged view of the rebuilt blade tip withdiscrete inclined tip squealer and film shelf notches; and

FIG. 11 is a flow diagram of a method of rebuilding a worn or damagedtip squealer.

DESCRIPTION OF THE PREFERRED EMBODIMENT AND BEST MODE

Referring now specifically to the drawings, a high pressure turbineblade of a gas turbine engine according to the prior art is illustratedin FIG. 1 and shown generally at reference numeral 10. The blade 10 ismounted directly downstream from a combustor (not shown) for receivinghot combustion gases. Blade 10 extends radially outwardly from a rotordisk, not shown, along a radial axis. Blade 10 includes a hollow airfoil12 and a dovetail 14 configured for being mounted in a correspondingdovetail slot in the perimeter of the rotor disk.

The blade 10 also includes an integral platform 18 disposed at thejunction of airfoil 12 and dovetail 14 for defining a portion of theradially inner flowpath for combustion gases. Blade 10 may be formed inany conventional manner, and is typically a one-piece casting.

Airfoil 12 preferably includes a generally concave pressure sidewall 20and an opposite, generally convex, suction sidewall 22 extending betweenopposite leading and trailing edges 24 and 26, respectively. Sidewalls20 and 22 also extend in the radial direction between a root 26 atplatform 18 and an outer tip 28, and are spaced apart over the entirespan of airfoil 12 to define at least one internal flow channel 30, seeFIG. 3, for channeling cooling air through the airfoil 12. The coolingair is typically bled from compressor (not shown) in any conventionalmanner.

The inside of airfoil 12 may have any configuration including, forexample, serpentine flow channels with various turbulators formedtherein for improving cooling air effectiveness, with the cooling airbeing discharged through film cooling holes 32 and trailing edgedischarge holes 34.

As seen in FIGS. 1 and 2, tip 28 preferably includes a tip cap 36integrally formed atop the radially outer ends of the pressure andsuction sidewalls 20, 22, respectively, where the tip cap 36 boundsinternal flow channel 30.

As is shown in FIG. 2, a first tip rib 38 preferably extends radiallyoutwardly from tip cap 36 between the leading and trailing edges of thepressure sidewall 20. A second tip rib 40 extends radially outwardlyfrom tip cap 36 adjacent suction sidewall 22. The first tip rib 38 isrecessed from the pressure sidewall 20 to form a tip shelf 42substantially parallel to tip cap 36 as in accordance with the prior artto improve cooling of tip 28.

Referring now to FIGS. 3 and 4, a pocket 44 is formed between the firsttip rib 38 and tip shelf 42 which promotes a recirculation of combustiongases. A plurality of cooling holes 46 are preferably provided along thelength of tip shelf 42 to provide a cooling film recirculation zone toassist in maintaining a cooling film near first tip rib 38.

As shown in FIGS. 3 and 4, the first tip rib 38 is inclined with respectto the radial axis of the blade, and a longitudinal axis of the secondtip rib 40 may be substantially parallel to the first tip rib 38, or maybe substantially parallel to the radial axis of the blade 10. Theabove-described structure is generally referred to as a “squealer tip.”Further aspects and details of the blade 10 and its alternativeembodiments are found in U.S. Pat. No. 6,672,829.

Referring now to FIG. 5, it is presumed that the blade 10 has beenplaced in use and has either been damaged or the tip 28 has worn to thepoint where replacement is required for continued use. In accordancewith one preferred embodiment of the method of the invention, thedamaged or worn portions of the tip 28 are ground off by conventionalmeans and the remaining tip cap 36 is dressed and otherwise prepared toreceive additional material. As is shown in FIG. 6, material to form newfirst and second tip ribs 38′, 40′ is welded to the tip cap 36 to form ablank from which the new blade tip 28′ will be constructed. The materialused to form the tip ribs 38′, 40′ may be the same alloy material as theairfoil 12, or an improved alloy. Brazing may also be considered analternative to welding.

The rebuild method continues by utilizing short, wedge-shaped electrodes“E” in spaced-apart relation along the length of the tip rib 38′ of theairfoil 12, as shown in FIG. 7. In an EDM process, notches 50A-50H, seeFIGS. 8 and 10, are formed in the first tip rib 38′ at the intersectionof the tip rib 38′ and the tip cap 36. FIG. 10 further illustrates thatthe notches 50A-50H define respective tip shelves 52A-52H.

After formation of the notches 50A-50H, cooling holes 54 are drilledfrom the tip shelves 52A-52H through the tip cap 36 and intocommunication with the hollow interior of the airfoil 12. Note that theholes 54 are preferably parallel to the back wall of the notches50A-50H.

As shown in FIG. 10, the resulting rebuilt blade 60 has an airfoil 12′with a squealer tip 28′ with several discrete tip shelves 52A-52H asdistinct from the continuous tip shelf 42 provided on the blade 10 shownin FIG. 1.

Of course, different blade sizes and shapes will determine the number,shape, size and spacing of the notches 50, and the number of holesdrilled within each notch—the embodiment described above being providedfor purposes of illustration. For example, the walls of the notches 50may be planar or non-planar, and the vertical sidewalls may divergeradially.

The method described above is shown schematically in FIG. 11.

The rebuild method described above, and the resulting rebuilt blade 10have a number of advantages. The EDM process itself is well-known andsimple to implement. Each EDM electrode “E” is short and therefore welladapted to being fitted into alignment with the surface curvature of theairfoil 12′. The total cooling surface area inside the notches 50A-50His greater than the corresponding cooling surface area inside thecontinuous tip shelf 42 of the airfoil 12 shown in FIG. 1. The overallcooling efficiency is thereby improved without any countervailingdisadvantages. The inclined back wall in each of the notches 50A-50H hasthe similar effect as the continuous inclined squealer tip in reducingtip leakage flow. Even though the notches 50A-50H are discrete and thusform a non-continuous squealer, the majority of the leakage flowreduction is maintained. The method described above can also be used inthe casting of the original blade 10 as an alternate to the cast-ininclined squealer tip.

A method of rebuilding the tips of continuous inclined squealer tipturbine blades, and the resulting rebuilt blade are described above.Various details of the invention may be changed without departing fromits scope. Furthermore, the foregoing description of the preferredembodiment of the invention and the best mode for practicing theinvention are provided for the purpose of illustration only and not forthe purpose of limitation—the invention being defined by the claims.

1. A method of constructing a turbine blade for a gas turbine engine,the blade being of the type including an airfoil having first and secondpressure and first and second suction spaced-apart sidewalls defining aninterior void and joined at a leading edge and a trailing edge, thepressure and suctions sidewalls extending from a root to a tip cap and asquealer tip including at least one tip rib extending outwardly from thetip cap, the method comprising the steps of: (a) casting the blade,including the airfoil; (b) forming a plurality of spaced-apart notchesin the airfoil proximate the tip on the pressure sidewall, wherein eachnotch is recessed with respect to the first sidewall to form arespective tip shelf, and further wherein each notch is defined by a topwall oblique with respect to a plane perpendicular to a longitudinalaxis of the blade and oblique to the first and second spaced-apartsidewalls, and the respective tip shelves are perpendicular to a surfaceof the first and second spaced-apart airfoil sidewalls; and (c) formingat least one hole in each tip shelf communicating with the interior voidof the airfoil for channeling cooling air from the interior void of theairfoil to thereby form a squealer tip.
 2. A method according to claim1, wherein the step of forming a plurality of spaced-apart notchescomprises the steps of: (a) providing EDM electrodes having apredetermined shape suitable for forming the notches; (b) applying theEDM electrodes to predetermined positions on the airfoil; and (c)utilizing the EDM electrodes to electrically discharge machine thenotches into the airfoil.
 3. A method according to claim 1, wherein thestep of forming at least one hole in each notch comprises the step ofdrilling the hole.
 4. A method according to claim 1, wherein the step offorming the at least one hole comprises the step of drilling a pluralityof holes in each notch.
 5. A method according to claim 1, wherein thestep of forming a plurality of spaced-apart notches in the airfoilcomprises the steps of: (a) providing EDM electrodes having apredetermined shape suitable for forming the notches; (b) applying theEDM electrodes to the airfoil in a spaced-apart,longitudinally-extending array between the leading edge and the trailingedge proximate a top edge thereof; and (c) utilizing the EDM electrodesto electrically discharge machine the notches into the airfoil.